The complexities of golf club design are known. The specifications for each component of the club (i.e., the club head, shaft, grip, and subcomponents thereof) directly impact the performance of the club. Thus, by varying the design specifications, a golf club can be tailored to have specific performance characteristics.
The design of club heads has long been studied. Among the more prominent considerations in club head design are loft, lie, face angle, horizontal face bulge, vertical face roll, center of gravity, inertia, material selection, and overall head weight. While this basic set of criteria is generally the focus of golf club designers, several other design aspects must also be addressed. The interior design of the club head may be tailored to achieve particular characteristics, such as the inclusion of hosel or shaft attachment means, perimeter weights on the club head, and fillers within the hollow club heads.
Golf club heads must also be strong to withstand the repeated impacts that occur during collisions between the golf club and the golf balls. The loading that occurs during this transient event can create a peak force of over 2,000 lbs. Thus, a major challenge is designing the club face and body to resist permanent deformation or failure by material yield or fracture. Conventional hollow metal wood drivers made from titanium typically have a uniform face thickness exceeding 2.5 mm to ensure structural integrity of the club head.
Players generally seek a metal wood driver and golf ball combination that delivers maximum distance and landing accuracy. The distance a ball travels after impact is dictated by the magnitude and direction of the ball's initial velocity and the ball's rotational velocity or spin. Environmental conditions, including atmospheric pressure, humidity, temperature, and wind speed, further influence the ball's flight. However, these environmental effects are beyond the control of the golf equipment designers. Golf ball landing accuracy is driven by a number of factors as well. Some of these factors are attributed to club head design, such as center of gravity and club face flexibility.
The United States Golf Association (USGA), the governing body for the rules of golf in the United States, has specifications for the performance of golf balls. These performance specifications dictate the size and weight of a conforming golf ball. One USGA rule limits the golf ball's initial velocity after a prescribed impact to 250 feet per second±2% (or 255 feet per second maximum initial velocity). To achieve greater golf ball travel distance, ball velocity after impact and the coefficient of restitution of the ball-club impact must be maximized while remaining within this rule.
Generally, golf ball travel distance is a function of the total kinetic energy imparted to the ball during impact with the club head, neglecting environmental effects. During impact, kinetic energy is transferred from the club and stored as elastic strain energy in the club head and as viscoelastic strain energy in the ball. After impact, the stored energy in the ball and in the club is transformed back into kinetic energy in the form of translational and rotational velocity of the ball, as well as the club. Since the collision is not perfectly elastic, a portion of energy is dissipated in club head vibration and in viscoelastic relaxation of the ball. Viscoelastic relaxation is a material property of the polymeric materials used in all manufactured golf balls.
Viscoelastic relaxation of the ball is a parasitic energy source, which is dependent upon the rate of deformation. To minimize this effect, the rate of deformation should be reduced. This may be accomplished by allowing more club face deformation during impact. Since metallic deformation may be substantially elastic, the strain energy stored in the club face is returned to the ball after impact thereby increasing the ball's outbound velocity after impact. Therefore, there remains a need in the art to improve the elastic behavior of the hitting face.
As discussed in commonly-owned parent patent U.S. Pat. No. 6,605,007, the disclosure of which is incorporated herein in its entirety, one way known in the art to obtain the benefits of titanium alloys in the hitting face is to use a laminate construction for the face insert. Laminated inserts for golf club heads are well-known in the art, where multiple metal layers of varying density are joined together to maximize the strength and flexural properties of the insert. The method used to join the layers together are critical to the life of the insert, as the repeated impacts with golf balls can eventually cause the insert to delaminate. In the art, laminated striking plate inserts for golf clubs, the bonding strength of the laminate is usually quite low, generally lower than the yield strength of the weakest material. As such, there remains a need in the art for additional techniques for effectively bonding together the layers of a laminate hitting face, particularly where all layers of the hitting face are titanium alloys.